The invention relates to controlling integration times of pixel sensors.
Referring to FIG. 1, a typical digital camera 12 uses an imager 18 to electrically capture an optical image 11. To accomplish this, the imager 18 typically includes an array 13 (see FIG. 2) of photon sensing, pixel sensors 20. During an integration time, or interval, each pixel sensor 20 typically measures the intensity of a portion, or pixel, of a representation of the image 11 that is focused (by optics of the camera 12) onto the array 13. To accomplish this, the pixel sensor 20 accumulates light energy that is received from the associated pixel and at the expiration of the integration interval, indicates (via an analog voltage, for example) the accumulated energy which also, in turn, indicates an intensity of light of the pixel.
The camera 12 typically processes the indications from the pixel sensors 20 to form a frame of digital data (which digitally represents the captured image) and transfers the frame (via a serial bus 15, for example) to a computer 14 for processing. For video, the camera 12 may successfully capture several optical images and furnish several frames of data, each of which indicates one of the captured images. The computer 14 may then use the frames to recreate the captured video on a display 9.
Referring to FIG. 2, the sensors 20 may be arranged in rows and columns. This arrangement allows column 22 and row 24 decoders to selectively retrieve the indications from the sensors 20 after the integration interval. The decoders 22 and 24 route the selected indications to signal conditioning circuitry 26 which might include, for example, analog-to-digital converters (ADCs) and circuitry to compensate for noise that is introduced by the sensors 20. The signal conditioning circuitry 26 may furnish the resultant data signals to an output interface 28 which includes circuitry for interfacing the imager 18 to other circuitry of the camera 12. A control unit 30 may coordinate the above-described activities of the imager 18.
The duration of the integration interval determines how long the pixel sensors 20 sense, or are exposed to, the optical image 11. In this manner, if the duration of the integration interval is too short, the pixel sensors 20 may be underexposed, and if the duration is too long, the pixel sensors 20 may be overexposed.
The camera 12 typically controls the duration of the integration interval based on the camera""s measurement of the brightness of the optical image 11. In this manner, for bright lighting conditions, the camera 12 uses a shorter duration (to prevent overexposure of the pixel sensors 20) than for low lighting conditions (to prevent underexposure of the pixel sensors 20). The camera 12 may measure the brightness of the image based on a histogram of sampled intensities.
The histogram represents a distribution of intensity levels of the pixel image over an available dynamic range (a range spanning from an intensity level of 0 to an intensity level of 255, for example). If the intensity levels are distributed over a large portion of the available dynamic range, then the image appears more vivid than if the intensity levels are distributed over a smaller portion of the available dynamic range. For example, a histogram 40 (see FIG. 3) for an image having an unacceptably low contrast exhibits a higher concentration of the lower intensities than a histogram 41 (see FIG. 4) for an image that has an acceptable contrast and thus, a larger dynamic range.
For purposes of determining the proper duration for the integration interval, the camera 12 may enter a calibration, or premetering, mode during which the camera 12 uses an iterative process to determine the duration. The camera 12 typically chooses a predetermined duration of the integration interval to sample intensities of the image 11 by using a small group of the pixel sensors 20. In this manner, the camera 12 may statistically evaluate a histogram of these intensities and based on this evaluation, upwardly or downwardly adjust the predetermined duration before sampling intensities again. The camera 12 continues the iterations until the camera 12 determines the duration of the integration interval is appropriate. However, this iterative process may consume a significant amount of time which may adversely affect the click-to-capture performance time of the camera. Furthermore, this delay may prevent the camera 12 from responding to changing lighting conditions in a timely fashion.
As described above, the camera 12 may set the duration of the integration interval based on the intensities indicated by a small group of the pixel sensors 20. However, the intensities sensed by this small group may not accurately represent the range of intensities of the optical image 11. For example, the small group may sense bright pixels of an otherwise dark image, and as a result, the camera 12 may use an integration interval that is too short to adequately capture the image.
Thus, there is a continuing need for a digital imaging system that addresses the to problems stated above.
In one embodiment, an imager includes groups of pixel sensing units and a control circuit. Each group of the pixel sensing units integrates photons from a different associated portion of an optical image over an integration interval for the group. The control circuit independently regulates the integration intervals for the groups.
In another embodiment, a method includes integrating photons from an optical image to capture a pixel image. An energy that is indicated by the integration is determined. The times for the energy to reach different predetermined threshold levels are measured, and a duration of the integration is regulated based on the measured times.